The present invention relates to imaging systems for use in a control system such as a vehicle headlamp control.
Headlamps illuminate a region in front of a vehicle allowing a driver to view the region when ambient light is insufficient. Headlamps also allow the vehicle to be seen by pedestrians and drivers of other vehicles. High beam headlamps provide even greater illumination and have a greater coverage region. However, high beam headlamps may blind drivers in oncoming vehicles and drivers in vehicles traveling in the same direction within the high beam coverage region. Traditionally, a driver has had to manually control turning headlamps on and off and switching between high beam and low beams.
One difficulty with manual control is that the driver may forget to turn headlamps on at dusk making the vehicle difficult to see. Another difficulty is that the driver may neglect to dim high beam headlamps for oncoming traffic or when approaching another vehicle from behind.
Previous attempts to automatically control the operation of vehicle headlamps have used sensors which provide a single output signal or a very small number of output signals to the associated control system. For example, a single output sensor has been used to sense ambient light for determining when to turn headlamps on or off. Also, a single output sensor has been used for determining when to dim automotive headlamps. Whereas a headlamp on/off control using a single sensor input has achieved limited success in automotive applications, a single sensor headlamp dimmer control is not currently offered because of its many shortcomings.
Array imaging sensors and various scanning techniques have been proposed, but even with the reduced costs made possible by today's electronics, these sensors and techniques have not produced satisfactory headlamp dimming and on/off control functions. Such sensing systems typically have hundreds of rows and columns of pixel sensors generating hundreds of thousands or even millions of pixels. At a typical video rate of 30 frames per second, this requires conversion and data processing rates in the millions of operations per second.
Headlamp on/off control can be based on ambient light levels. Headlamp dimmer control can be based on recognizing the headlamps from oncoming vehicles and the tail lamps of vehicles approached from behind. Since the resolution required to detect ambient light levels and to detect headlamps and tail lights is less than required for traditional images, a smaller imaging array, and hence, slower processing electronics, may be used.
In order to distinguish red tail lamps from other lights, the imaging system must produce readings in at least two different color bands. The first of two methods usually used to sense colors with an image sensor has been to cover one-third of the pixel sensing sights in the imager with a red or red complement filter, one-third of the pixels with a blue or blue complement filter, and one-third of the pixels with a green or green complement filter. This is often done, for example, by placing alternating red, green, and blue stripes over columns of pixels. Each pixel site registers one color and interpolation is used to supply the two missing colors at each pixel sight.
When coupled with a low resolution imager, this technique for sensing color creates a problem. Due to the optics used, the projected image of a headlamp or tail light viewed by the imaging sensing array is very small, probably smaller than the resolving power of the lens. This projected image will be referred to as a dot. When pixel spacing is significantly smaller than the dot size projected by the lens, a portion of a dot of a particular color may not always strike a sensor sight of that color. As the pixel size or area of optical coverage per pixel is increased due to a corresponding reduction in the number of pixels, the voids between the like colored pixel sights become larger unless a complicated interdigitated pixel pattern is used. Even if readout of a particular color is not completely lost by having the entire dot image projected on a pixel of another color or colors, the readout will be coarse depending on what portion of the dot strikes a pixel. Since distinguishing a color is usually a matter of determining balance between two or more color components and not just determining the presence or absence of a particular color component, when the small spot of light in the projected image of a headlamp or tail light falls more on one pixel of one color than another, the measured balance is altered accordingly.
A further disadvantage with this method results from dyes used to implement the color filters. The dyes are normally organic and are subject to degradation from thermal and light exposure. Since the dye sits directly over individual pixel sites, the energy from a strong light source, such as the sun, is focused by the lens system directly onto the dye.
A still further problem with this method is that having the color filter dye applied to and precisely registered with the pixel sensor sight on the image sensor is expensive. The cost of adding color filters directly on the pixel sensor may be as expensive as the silicon image sensing chip itself.
A second method for imaging color splits light from the image into red, green, and blue components which are projected onto separate image sensors, each of which measures its respective color filtered image. This requires a complicated optical arrangement and three separate image sensors. The color separation technique often utilizes mirrors which selectively reflect one color and transmit the complementary color. These optical arrangements normally require widely separated non-planar image sensor sights making it difficult, if not impractical, to place the three sensors on a common silicon substrate or even in a common package. This technique presents a three-fold problem. A single sensor array cannot be used, a single silicon chip cannot be used, and a single package cannot be used.
What is needed is a cost effective imaging system to be used in, for example, a headlamp control system. To limit costs and complexity in the optics, the sensor array, the processor, and processor interface, a minimal number of pixels, preferably in a range which would be considered too small for satisfactory pictorial image presentation, should be used. The imaging system should not use spectral filtering that would place dyes or color-selecting materials in the focal point of the lens system. The imaging system should supply signals appropriate for determining headlamp dimming control, headlamp on/off control, or both. The imaging system should also be protected against excessive light or heat damage.